Polyurethane foams are generally obtained by reacting polyisocyanates with polyols, the latter containing active hydrogen atoms. Such foams are widely used in industry and are, for example, employed for insulation purposes, for the production of structural elements and for upholstery and elastomeric purposes.
For many of the applications to which such foams are put, it is desirable that the foams have an open cell structure. Foams with closed cells exhibit decreased elasticity or resiliency and, moreover, have a tendency to shrink after the production. It is, moreover, desired that the load bearing capability or the hardness of open cell, resilient foams should be variable over a wide range. The desired hardness can be imparted to a foamed body by increasing the degree of cross-linking, by increasing its unit weight or density or by increasing the isocyanate content of the formulation. From a technical and economical point of view, an increase of the degree of cross-linking is the most advantageous of these three possibilities. This is so because in order to increase the unit weight (density) or the isocyanate content, larger amounts of starting materials are required which, of course, renders the production of the foams less economical. Further, an increase in the isocyanate content is disadvantageous because foams with increased isocyanate content have greater tendency to flammability and toxicity. Moreover, the "feel" of the foams becomes harder with increased isocyanate content.
According to the present state of the art, an increase in the degree of cross-linking of a polyurethane foam formulation is obtained by adding low molecular, polyfunctional compounds which are soluble in the active hydrogen atom-comprising polyols. This means that the cross-linking agents can react immediately upon initiation of the foam forming reaction. The possibility of increasing the cross-linking density is, however, limited in such a procedure. This is due to the fact that, dependent on the other ingredients of the foaming formulation, the cross-linking density at the time at which the foams have reached their maximum rising height -- to wit, the time at which usually the cell opening is initiated -- is already too high to permit effective cell opening.
Foams are then obtained which, it is true, have a high cross-linking degree. These foams, however, have a large proportion of closed cells. In some instances, all the cells are then closed. Such foams exhibit decreased flexibility and resiliency and, due to the fact that the cells are closed, have the tendency to shrink after the production.